Thirty feet down, at the bottom of the murky Sacramento River, swims a huge, ugly and, according to marine biologists, stupid prehistoric fish whose value far exceeds its physical attractions.

It is the white sturgeon, which is actually black and sometimes grows to 500 pounds. It is the focus of undercover agents and black marketeers, and it could become a new challenge to the Soviet Union.

A full third of the weight of a spawning female sturgeon may be composed of roe that can be made into caviar of exquisite quality. Its smoked flesh is a delicacy worth up to $35 per pound. The roe doesn't quite match the famed Russian or Iranian beluga caviar, because Americans haven't learned how to process it, but it is close.

The best Iranian caviar goes for $395 per troy pound (12 ounces). American caviar is about $95 for the same amount.

None of that domestic caviar, however, comes from the Sacramento River or its tributaries, at least not legally. Decades ago, Sacramento exported caviar to Europe, but over-fishing and pollution virtually destroyed the fishery, and since then only sport fishing has been allowed. It is illegal to sell either caviar or sturgeon flesh from the Sacramento, and most domestic caviar comes from the Columbia River in Oregon.

The demand has spawned a burgeoning black market in the San Francisco Bay area for both caviar and smoked sturgeon. State agents arrested six men a little over a year ago in a case officials charged involved as many as 60 fishermen and 35 to 40 restaurants, bait shops, wholesale fish dealers and fish markets. Agents are certain the same activity continues today.

In fact, the delta region where most of the spawning fish are caught is becoming known as the Mekong Delta because of the amount of illegal activity in the area.

State and federal game wardens have even traded their traditional green uniform for tuxedos and boiled shirts to circulate undercover at San Francisco political fund-raising events, trying to discover where all the expensive caviar is coming from.

One poacher, who asked that his name not be used, said he has made between $3,000 and $4,000 to supplement his income during the spawning run, putting as many as six deep-sea poles in stocks hidden in foliage along the river. Hooks almost as big as baling hooks are baited with shrimp, and cowbells are suspended from the rod tips.

The poacher waits for the bells to ring. When they do, he wrestles his catch to captivity. Then he calls a San Francisco number that brings a black marketeer racing toward his catch any time of day or night.

The poacher must keep the fish alive, noosed in the water, until the buyer arrives to ensure that the roe will be fresh. The poacher reportedly has received $5 to $6 a pound for the roe. A 200-pound fish may produce 60 to 70 pounds of caviar, and the poacher keeps the meat, which he also can sell on the black market for $5 a pound.

In an attempt to cool the black market, the state has authorized commercial breeders to attempt to create a commercial fishery, far behind the Russians, who operate 24 full-fledged hatcheries. The Russians have been researching sturgeon breeding for 20 years.

Because of the overthrow of the shah of Iran and the collapse of the Iranian sturgeon industry, caviar has become a major Soviet export.

But the Americans scored a major coup in the form of Sergei I. Doroshov, once considered the Soviet Union's top authority on marine aquaculture. On the way home from a mid-1970s trip to Cuba to study the lowly mullett, Doroshov and his family stopped in Rome and defected.

Today, Doroshov is at the University of California's Davis campus, 15 miles from the Sacramento River, helping the university develop perhaps the most advanced research program in the United States on anadromous fish, those that swim upstream to spawn. The university is helping five sturgeon farmers get under way.

Today, according to one, the program is "still in the R-and-D stage." No one has gotten sturgeon to mate in captivity; their roe and sperm, called milt, are removed by sport fishermen and mixed by hand.

The results, however, have been encouraging. A wild female may drop a half million to a million eggs, but only a half dozen of those will live to adulthood. The domestic breeders today are achieving survival rates of 30 to 50 percent, meaning a big female may produce as many as half a million live young.

Under the terms of their permits, the breeders must release half the hatchlings back into the river to replenish the fishery. Those that are released show an encouraging survival rate. Once they are past the hatchling stage, they grow spiny skins that cause predators to spit them out.

Domestic sturgeon grow quickly. One commercial operator recently showed off fish born in April that were seven inches long in August.

Commercial production, experts say, is probably six years away. The fish are marketable when they are between two and three years old. The females don't produce eggs until they are about seven years old, and since only one or two of the five potential breeders want to feed the fish for the extra five years, caviar probably always depend on wild stock. Nonetheless, state fish and game officials feel the program will eventually add healthy numbers of roe-bearing females to the wild population, to the point that commercial fishing can be resumed on the Sacramento River.